Method and apparatus for controlling continuous lye-washing in preparing pulp for papermaking

ABSTRACT

A METHOD FOR INSPECTION OF CONTINUOUS LYE-WASHING OR LIGESTION IN THE PREPARATION OF PAPER PULP, AND AN APPARATUS FOR CARRYING OUT THE METHOD, WHEREIN THIS INSPECION IS DEFINED BY THE OBTAINING OF INFORMATION AS TO THE ATTENUATION OF AN ACOUSTIC WAVE PASSING THROUGH THE REACTION MEDIUM OF THE DIGESTER, CREATED BY AN ULTRASONIC BEAM EMITTED BY A TRANSDUCER AND PICKED UP BY A SECOND TRANSDUCER FORMING A RECEIVER, BOTH OF WHICH ARE MOUNTED AT A FIXED DISTANCE, THEIR COMMON AXIS BEING AT 30* TO A DIAMETER OF THE DIGESTER; BOTH RESONATING ON THE SAME FREQUENCY, WHICH IS HIGH ENOUGH AND LOW ENOUGH TO AVOID STRAY WAVES AND TO PERMIT DESPIT SEVERAL FACTORS OF RECEIVING A SIGNAL WHICH IS LARGE IN COMPARISON WITH STRAY SIGNALS.

Oct. 12, 1971 R c RF 3,611,793

E METHOD ANDAPPARATUS FOR CONTROLLING CONTINUOUS LYE-WASHING IN PREPARING PULP FOR PAPERMAKING Filed July 25, 1968 2 Sheets-Shoat; 1

Figure/l Oct. 12, 1971 CERF 3,611,793 METHOD AND APPARATUS CONTROLLING CONTINUOUS E-WASHING IN PREPA G PULP FOR PAP KI LY Filed July 25, 1968 Sh s-Shoet 2 Fi-gurer2 United States Patent Office 5 Int. Cl. G01n 24/00 US. Cl. 73-675 6 Claims ABSTRACT OF THE DISCLOSURE A method for inspection of continuous lye-washing or digestion in the preparation of paper pulp, and an apparatus for carrying out the method, wherein this inspection is defined by the obtaining of information as to the attenuation of an acoustic wave passing through the reaction medium of the digester, created by an ultrasonic beam emitted by a transducer and picked up by a second transducer forming a receiver, both of which are mounted at a fixed distance, their common axis being at 30 to a diameter of the digester, both resonating on the same frequency, which is high enough and low enough to avoid stray waves and to permit despite several factors of receiving a signal which is large in comparison with stray signals.

INTRODUCTION AND BACKGROUND OF THE INVENTION The present invention relates to a method and apparatus for controlling continuous lye-washing in preparing pulp for papermaking.

The operation of lye-washing or digestion in the preparation of paper pulp consists in cooking wood shavings in a white liquor, solution of caustic soda and sodium sulphate, which has the property of hot-dissolving the non-cellulose components of the wood while leaving the cellulose fibers practically unattacked. Various known processes permit of carrying out this cooking continuous' ly, the shavings and the liquor being introduced permanently at the upper end of the digester, the crude pulp being extracted at the lower end in a continuous but regulatable delivery.

According to the delivery of the shavings at the entry of the digester, at a given instant and at a given level in the digester there will be a difierent condition of the reaction mixture and more especially a more or less large fraction of unattacked or slightly attacked shavings.

Now the determination of the condition of the reaction mixture at a given moment and at a given level is diflicult to efiect, since a pressure of about 12 kg./sq.cm. and a temperature of the order of 200 C. prevail in the digester and withdrawals or other means for example can not be utilized or are poorly eifective.

However, in order eifectively to regulate the running of the digester and especially the delivery of the shavings at the entry, it would be advantageous to be able to determine the condition of the reaction mixture in the digester at one or more levels.

It is an objective in the present invention to provide a method which solves this problem of supervision of the condition of the reaction mixture at any desired level by obtaining information as to the attenuation of an acoustic wave passing through the medium to be studied or determined, this wave not necessarily having a frequency lim ited to the audible range and more especially being capable of being ultrasonic.

3,611,793 Patented Oct. 12, 1971 BRIEF SUMMARY OF THE INVENTION In accordance with the present invention, the solution of this supervision problem results from realization of various measurements effected by the inventor in a digester used for the preparation of paper pulp. An ultrasonic beam passing through the mixture at several meters from the top of the digester and in a horizontal plane was picked up by a receiver which supplied an electric signal of the same frequency as the wave and the amplitude of which was slighter as the absorption of the medium passed through was greater. Measurements were effected at regular intervals, while the delivery of shavings was diminished. The fraction of fresh or slightly attacked shavings decreased in the digester and especially at the'level where the measurements were effected. The voltage read on the reading apparatus increased regularly, then remained constant. The observations which were made established that the attenuation of the ultrasonic beam varies sufliciently according to the composition of the reaction mixture to permit supervision thereof and consequently supervision and regulation of the running of the digester.

FURTHER DESCRIPTION AND ADVANTAGES OF THE INVENTION The method for the measurement of the attenuation of an acoustic wave in a material medium is thus based upon the principle of producing in this medium a system of acoustic waves between an emitter and a receiver effecting the reciprocal transformation of electric waves into acoustic waves, a measuring apparatus connected to the receiver permitting of determining the amplitude of the acoustic waves received by the latter.

However, this problem possesses particular difficulties connected with the great absorption of the reaction me dium, which at certain stages in its evolution contains shaving fragments, and with the agitation and lack of homogeneity of this medium, as well as with the vibrations of the digester and other accessory apparatuses, produce significant stray signals in the receiver. It is also necessary to limit the electric power necessary for the emitter and the transmission of acoustic energy to the wall of the digester and through this wall to the receiver. The distance between the emitter and the receiver is fixed.

The energization of the emitter is continuous throughout a measurement; an emitted wave must not encounter a wave reflected on the receiver. The frequency of the waves is fixed so as to be large enough, as a result of the absorption of the medium, to avoid stationary waves between the emitter and the receiver and small enough for it to be possible, in view of the inhomogeneity of the medium, its high absorption value, the magnitude of the stray signals and despite the limit imposed on the power applied to the emitter, for a signal to be received which is large in comparison with the stray signals. By way of substantiation, it would not be advantageous to depart too much from a frequency of 20 kHz.

For its part, the acoustic energy transmitted by the emitter to the wall of the container is limited, as is that between the wall and the receiver, in such a manner that variations of the amplitude of the Wave received characterize the variations of the attenuation of the medium with a maximum of sensitivity.

BRIEF DESCRIPTION OF THE VIEWS IN THE DRAWINGS With these and other objects in view which will become apparent in the following detailed description, the present invention will be clearly understood in connection with the accompanying drawings, in which:

FIG. 1 is a horizontal section of the digester including the common axis of the emitter and receiver;

FIG. 2 is a view showing the mounting of a transducer (emitter or receiver), the manner of fixing of its support, the active cylinder and its support, in greater detail, and

FIG. 3 is a diagram of the mounting of a millivoltmeter branched on the windings of the magneto-striction bars of the receiver, and of an inductance.

DETAILED DESCRIPTION Referring to the drawings, according to the present invention, the emitter and the receiver are coaxial magnetostriction transducers, to the active part of which the form of a cylinder of revolution is advantageously given. Both resonate at the same frequency. The common axis of the emitter e and of the receiver r is at 30 to the diameter of the digester. This arrangement, which was used in the experiments, has been adopted as shown by FIG. 1 to prevent the ultrasonic beam from meeting certain elements disposed within the digester.

As may be seen from FIG. 2, which shows in greater detail the mounting of a transducer (emitter or receiver) fixed n the wall p of the digester by its support s, the active cylinder c and its support s are turned from one single block of material.

The support s is provided with a constriction a with the purpose of limiting the acoustic energy transmitted by the emitter to the wall of the container. Moreover this support is fixed by screws (not shown) and through the intermediary of joints to the sleeve m which is itself fast with the wall of the digester.

The forward part of the cylinder c dips directly into the reaction medium which fills the sleeve m. The block from which the cylinder 0 and its support s are turned is of stainless steel.

Under these circumstances, the corrosive medium into which the front face of the transducer is dipped is in contact only with stainless steel, with the exception of the joint 1'.

On the rear face of the transducer, that is to say to the exterior of the digester, eight commercial magneto-striction bars are screwed parallel with the axis of the cylinder, the frequency of which bars is adapted to the cylinder. These bars are not represented; they are easy to remove and can conveniently be cooled if this should be necessary.

The transducers provided with their magneto-striction bars resonate at a frequency close to 20 kHz.

A commercial generator of sinusoidal electric signals operating at 20 kHz. and in a frequency range around this value, further provided with a device for the polarization of the magneto-striction bars, is connected directly to the windings of the latter mounted in series. In the aforementioned experiments, the windings constituted a layer of insulated wire of 1 mm. in diameter. The power of the generator can be less than 1 kw.

A millivoltmeter v admitting the frequency of 20 kHz. is branched on the windings of the bars of the receiver in accordance with the diagram in FIG. 3. These windings b are also mounted in series. The polarization of the receiver bars is effected by a direct current of the order of 7.5 amp, delivered either by a battery or by the polarization circuit of the generator if the latter is capable of simultaneously supplying the bars of the emitter and of the receiver.

The windings of the bars of the receiver constituted in the experiment a layer of wire of 65/100 mm. in diameter.

An inductance iplaced between the direct current source and the voltmeter prevents the latter from being shortcircuited. A trap circuit could also be used for this purpose.

The above numerical data is obviously only indicative and can be modified to a certain degree.

It is clear that the method can be applied to other problems of supervision of a material medium contained in a reservoir.

I claim:

1. A method for the supervision of continuous lye-washing or digestion in the preparation of paper pulp by measurement of the attenuation of an acoustic wave passing through the material medium in a container to be-determined, comprising the steps of:

producing in said medium acoustic waves between an emitter of acoustic waves and a receiver of acoustic waves effecting reciprocal transformation of electric waves into said acoustic waves, and over a fixed transmission distance, the acoustic waves being attenuated by said medium,

repetitively energizing said emitter, the period of said energization not exceeding a limit of the order of magnitude of one second and being continuous through said period,

fixing a combination of wave frequency and acoustic energy of the waves such that the frequency is sufficiently high to avoid stationary waves between said emitter and said receiver and low enough to permit receiving a signal which is large in comparison with stray signals, and this magnitude of said frequency cooperating with a limit on the energy of the waves corresponding to the limit of l kw. imposed on electric power applied to said emitter, further taking into account the lack of homogeneity of the medium, the high value of the absorption, and the magnitude of said stray signals, and

regulating the composition of the medium according to the attenuation of said acoustic waves.

2. The method, as set forth in claim 1, wherein:

said container is a digester having a circular cross-section, and

orienting said emitter and receiver in a common axis about 30 relative to a diameter of said digester.

3. An apparatus for the supervision of continuous lyewashing or digestion in the preparation of paper pulp, comprising:

an emitter and a receiver comprising coaxial spaced magneto-striction transducers,

both of said transducers resonate at the same frequency,

a digester containing a reaction medium,

said emitter and receiver having a common axis oriented at 30 to the diameter of said digester on which the receiver is mounted,

each of said transducers including an active part and a support which are turned from one single block of stainless steel,

a sleeve to which said support is fixed, said sleeve being connected with a wall of said digester,

said support is formed with a constriction,

said active part of said transducers dip directly into said reaction medium,

magneto-striction bars mounted on a rear face of said transducers at the exterior of said digester,

the resonance frequency of said transducers is about 20 kHz,

a generator of sinusoidal electric signals equipped with a polarization device is connected directly to said magneto-striction bars of said emitter which are mounted in series,

windings for said magneto-striction bars,

a millivoltmeter admitting the frequency of 20 kHz. is branched on said windings of said magneto-striction bars of said receiver, which are also mounted in series,

a direct current source for said magneto-striction bars of said receiver for polarizing said magneto-striction bars,

means for separating said millivoltmeter from said direct current source to prevent said millivoltmeter References Cited from being short-circuited, and UNITED STATES PATENTS means for regulating the composition of the medrum in accordance with the attenuation of the said acous- 2,505,367 5/ 1950 Melllliel' tic waves, 5 2,755,662 7/1956 Swengel 4. The apparatus, as set forth in claim 2, wherein: 68,524 10/1956 Beard 7367 'd d t r t e sa'd olar' at d gfii ji fi 1s 1 p 12 ewe RICHARD c. QUEISSER, Primary Examiner 5. The apparatus, as set forth in claim 2, wherein: A KORKOSZ, Assistant Examiner said separating means is an inductance. 10

6. The apparatus, as set forth in claim 2, wherein: U.S. C1. X.R.

said separating means is a trap circuit. 162--49, 50 

